The present invention relates to implantable electrical devices and, more particularly, an implantable cardiac device that incorporates a magnetoresistive-based motion sensor which is adapted to sense the motion or acceleration of the body of the patient in which the device is implanted.
Implantable cardiac devices, such as pacemakers and implantable cardioverter defibrillators (ICDs), have become increasingly sophisticated over the past several years. These devices are now capable of sensing the performance of the heart and responsively applying therapeutic electrical stimulation to the heart that is tailored to correct the heart""s performance.
In fact, current generation pacemakers are capable of sensing the activity level of the patient and then tailoring the delivery of pacing pulses to the patient""s heart to match the activity level of the patient. For example, if the pacemaker ascertains that the patient is more active, the pacemaker increases the pacing rate so that the patient""s heart beats more rapidly to provide an increased flow of blood to the patient. Conversely, when the patient is at rest, the pacemaker decreases the pacing rate so as to maximize the battery life of the pacemaker and also so that the heart rate of the patient more closely mirrors normal function of the heart when the patient is at rest.
Typically, pacemakers and other implantable cardiac devices that provide therapy based at least in part on the activity of the patient, incorporate some sort of an activity sensor. Typically, the activity sensor is comprised of an accelerometer that provides a signal that is indicative of the activity level of the patient. The accelerometer is generally positioned within the implantable cardiac device casing and the accelerometer provides a signal which is indicative of the acceleration experienced by the casing. It is, of course, understood that the greater the activity level of the patient, the more the casing is accelerated. Hence, the accelerometer is capable of providing a signal which is indicative of the activity level of the patient.
Typically, accelerometers that are used in implantable cardiac devices incorporate some sort of piezo-electric sensor. One such accelerometer is described in U.S. Pat. No. 5,425,750 to Moberg. This sensor incorporates a cantilevered beam with a weight mounted on the cantilevered end of the beam. The surface of the beam is coated with a piezo-electric crystal polymer. Acceleration of the casing containing the accelerometer results in the cantilevered beam bending in response to the acceleration. The piezo-electric crystal is mechanically deformed by this bending and thereby produces an electrical signal that is proportionate to the mechanical deformation of the crystal. This electric signal can be amplified and used to provide an indication of the activity level of the patient.
One difficulty associated with using these piezo-electric sensors is that the piezo-electric material is often very expensive. This is particularly true for the very sensitive sensors that have to be used in implantable cardiac devices. The accelerometer as a whole cannot be very large in size as it has to be positioned in the limited confines of an implantable cardiac device casing. Hence, the piezo-electric material must also be relatively small in size which requires the piezo-electric material to be very sensitive in order to be able to provide an electrical signal that is reflective of the acceleration of the pacemaker casing. These types of piezo-electric materials are very expensive and increase the overall cost of the implantable cardiac device.
Also, piezo-electric acceleration sensors are inherently AC coupled. This type of sensor only produces an output proportional to the beam""s rate of change of bending and cannot be used to sense the position of the patient""s body. Another difficulty associated with the piezo-electric acceleration sensors is that, even though very sensitive piezoelectric materials can be used, the limited amount of space that is taken up by the piezo-electric sensor can still be quite considerable. As the implantable cardiac device casing is implanted within the body, it is desirable to minimize the size of each of the components that are positioned within the casing so as to reduce the overall size of the implantable cardiac device. Unfortunately, the piezo-electric based acceleration sensor must have a certain minimum amount of surface area in order for the material to mechanically deform sufficiently so as to provide a usable signal indicative of the patient""s activity level. Consequently, the minimum size of an accelerometer of this type is comparatively large and is not subject to significant reduction in size.
Moreover, the piezoelectric crystals used in these types of activity sensors are under continuous repeated stress. This results in fatigue in the crystalline structure that can, ultimately, result in the activity sensor ceasing to work. It will, of course, be appreciated that replacement of inoperative activity sensors in implanted cardiac devices is impractical if not impossible due to the invasiveness of the procedure.
Hence, there is a need for a sensor which is capable of detecting the activity level of the patient and providing a signal indicative thereof which is both small in size and made of inexpensive components. To this end, there is a need for a sensor which does not require the use of a large amount of surface area, does not require the use of very sensitive piezo-electric materials, and is more resistant to material fatigue related failure.
The aforementioned needs are satisfied by the sensor of the present invention which is adapted for use with an implantable electrical device. The sensor of the present invention incorporates a magnetoresistive sensor and a magnet that are mounted so as to be movable with respect to each other. Relative movement between the magnet and the magnetoresistive sensor produces a change in the resistance value of the magnetoresistive sensor. This change in the resistance value can be sensed by the application of a voltage to the sensor so that the resulting output signal is indicative of the activity of the patient.
In one aspect of the invention, the magnetoresistive sensor is mounted to a substrate and a permanent magnet is mounted on a bendable cantilevered beam that is attached to the substrate so as to position the permanent magnet in proximity to the magnetoresistive sensor. Movement of the substrate will result in the cantilevered beam bending and vibrating. The movement of the cantilevered beam results in the permanent magnet changing its relative position with respect to the magnetoresistive sensor thereby causing the resistance of the magnetoresistive sensor to change.
In one embodiment of the invention, the magnetoresistive sensor is comprised of a giant magnetoresistive (GMR) sensor that provides a differential output voltage which is indicative of the change of the sensed magnetic field. The output signal varies in both amplitude and frequency and both of these variables can be used by the control unit of an implantable cardiac device as inputs for adjusting and optimizing the delivery of therapeutic electrical stimulation to the heart of the patient.
In another aspect of the invention, an implantable cardiac device is provided which includes a magnetoresistive sensor that provides a signal to a controller which is indicative of the movement of the sensor. The sensor is adapted to be implanted within the body of the patient so that movement of the patient will result in movement of the sensor thereby inducing the sensor to provide the signal to the controller which is indicative of the movement of the patient.
In one embodiment, the sensor is comprised of a magnetoresistive sensor that is fixedly mounted on a substrate and a permanent magnet that is mounted on a cantilevered beam so as to be positioned adjacent the magnetoresistive sensor. The sensor is adapted to be positioned within the body of the patient so that movement of the patient results in the magnet attached to the cantilevered end of the beam moving with respect to the magnetoresistive sensor. The magnetoresistive sensor thereby provides a signal to the controller which is indicative of the movement.
In one aspect of this embodiment of the invention, the magnetoresistive sensor is comprised of a giant magnetoresistive (GMR) sensor which provides an analog voltage to a voltage controlled oscillator. The voltage controlled oscillator provides an output signal to a counter which counts the oscillation signal provided by the voltage controlled oscillator. The output of the counter is then sampled by the implantable cardiac device""s processor which provides the processor with a signal that is indicative of both the amplitude and the frequency of the movement sensed by the sensor.
It will be appreciated that the processor can then use this information to change the therapeutic electrical stimulation that is being applied to the heart of the patient. For example, if the frequency and amplitude of the signal from the sensor suggests the patient is engaged in more rigorous physical activity, the processor can use this information to increase the pacing rate of a pacemaker to enable the heart to pump more blood to the body""s extremities. Conversely, when the acceleration signal indicates that the patient is at rest, the processor can then reduce the pacing rate so as to conserve battery power and also so as to have the heart performance more closely mirror the normal heart performance when the patient is at rest.
By using a magnetoresistive sensor as opposed to a piezo-electric element, the preferred activity sensor can be smaller in size, less expensive to manufacture and less prone to subsequent failure. These and other objects of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.